1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a belt having parts of the tension and compression sections thereof made from different rubber compositions to exhibit both a) resistance to wear in high and low temperature environments and b) resistance to oil penetration.
2. Background Art
In recent years, there has been an increasing demand for the toothed power transmission belt. The toothed power transmission belt has several advantages. First of all, it affords positive interaction with associated pulleys. The teeth provide slip-free driving, which cannot be accomplished with the conventional flat edge belt and V-belt. Toothed belts are also desirable in that they require no lubrication as is necessary with drives utilizing gears and/or chains.
The toothed belt has been particularly desirable in the automobile industry. It is commonly used in power transmission for overhead cam shafts (OHC) in automobiles. In the interest of conservation, the automobile industry has been designing smaller and more energy efficient automobiles. Designers strive to minimize the size of engines as well as engine compartments. These engines often operate at high speeds which results in substantial heat generation in the compartment therefor. Accordingly, belts for automobile engines must be constructed to withstand the high operating temperatures as well as the cold conditions that may be encountered at start-up.
It is conventional to construct such toothed belts primarily from chloroprene rubber. Chloroprene rubber, however, has a tendency to crack in high temperature environments. This is particularly a problem at the hardened belt tooth root. To overcome this problem, efforts have been made to improve the heat resistance of chloroprene rubber. While some improvement has been made, chloroprene rubber has certain inherent limitations at high temperatures. Consequently, satisfactory results with power transmissions have not been realized to date with chloroprene rubber.
As a result, there has been an increased effort to change from chloroprene rubber to rubbers with better heat resistance, and more particularly rubbers whose main chain has been highly or completely saturated. Exemplary of such rubbers are chlorosulfonated polyethylene rubber and hydrogenated nitrile rubber. Japanese Patent Laid Open Publication No. 1598227/1987 discloses a toothed belt utilizing hydrogenated nitrile rubber.
In using chlorosulfonated polyethylene rubber, designers of belts deal with two competing objectives; that of improving temperature resistance and oil resistance. Heat resistance, cold resistance and oil resistance are all affected by the chlorine content of the polymer. By controlling the chlorine content to improve the temperature resistance of the belt, the oil resistance is compromised, and vice versa.
Hydrogenated nitrile rubber also has some inherent drawbacks. First of all, it is very expensive. Secondly, it is a difficult composition to work with.